Structural Insights into Bound Water in Crystalline Amino Acids: Experimental and Theoretical (17)O NMR

High-Resolution 17O Solid-State NMR as a Unique Probe for Investigating Oxalate Binding Modes in Materials: The Case Study of Calcium Oxalate Biominerals

Oxalate ligands are found in many classes of materials, including energy storage materials and biominerals. Determining their local environments at the atomic scale is thus paramount to establishing the structure and properties of numerous phases. Here, we show that high-resolution 17O solid-state NMR is a valuable asset for investigating the structure of crystalline oxalate systems. First, an efficient 17O-enrichment procedure of oxalate ligands is demonstrated using mechanochemistry. Then, 17O-enriched oxalates were used for the synthesis of the biologically relevant calcium oxalate monohydrate (COM) phase, enabling the analysis of its structure and heat-induced phase transitions by high-resolution 17O NMR. Studies of the low-temperature COM form (LT-COM), using magnetic fields from 9.4 to 35.2 T, as well as 13C-17O MQ/D-RINEPT and 17O{1H} MQ/REDOR experiments, enabled the 8 inequivalent oxygen sites of the oxalates to be resolved, and tentatively assigned. The structural changes upon heat treatment of COM were also followed by high-resolution 17O NMR, providing new insight into the structures of the high-temperature form (HT-COM) and anhydrous calcium oxalate α-phase (α-COA), including the presence of structural disorder in the latter case. Overall, this work highlights the ease associated with 17O-enrichment of oxalate oxygens, and how it enables high-resolution solid-state NMR, for "NMR crystallography" investigations.

Automatic fitting of multiple-field solid-state NMR spectra

The NMR lineshapes produced by half-integer quadrupolar nuclei are sensitive to 11 distinct fit parameters per inequivalent site. To date, automatic fitting routines have failed to replace manual parameter insertion and evaluation due to the importance of local minima and the need for fitting multiple-field magic-angle spinning (MAS) and static spectra simultaneously. Herein we introduce a new tool, AMES-Fit (Automatic Multiple Experiment Simulation and Fitting), to automatically find the global best-fit simulation parameters for a series of multiple-field NMR lineshapes. AMES-Fit uses an adaptive step size random search algorithm to dynamically probe parameter space and requires minimal human input. The best fits are obtained in a few minutes of computation time that would otherwise have required several person-hours of work. The program is freely available and open-source.

Electron and Spin Delocalization in [Co6 Se8 (PEt3 )6 ]0/+1 Superatoms

Molecular clusters can function as nanoscale atoms/superatoms, assembling into superatomic solids, a new class of solid-state materials with designable properties through modifications on superatoms. To explore possibilities on diversifying building blocks, here we thoroughly studied one representative superatom, Co6 Se8 (PEt3 )6 . We probed its structural, electronic, and magnetic properties and revealed its detailed electronic structure as valence electrons delocalize over inorganic [Co6 Se8 ] core while ligands function as an insulated shell. 59 Co SSNMR measurements on the core and 31 P, 13 C on the ligands show that the neutral Co6 Se8 (PEt3 )6 is diamagnetic and symmetric, with all ligands magnetically equivalent. Quantum computations cross-validate NMR results and reveal degenerate delocalized HOMO orbitals, indicating aromaticity. Ligand substitution keeps the inorganic core nearly intact. After losing one electron, the unpaired electron in [Co6 Se8 (PEt3 )6 ]+1 is delocalized, causing paramagnetism and a delocalized electron spin. Notably, this feature of electron/spin delocalization over a large cluster is attractive for special single-electron devices.

Improving the accuracy of GIPAW chemical shielding calculations with cluster and fragment corrections

Ab initio methods for predicting NMR parameters in the solid state are an essential tool for assigning experimental spectra and play an increasingly important role in structural characterizations. Recently, a molecular correction (MC) technique has been developed which combines the strengths of plane-wave methods (GIPAW) with single molecule calculations employing Gaussian basis sets. The GIPAW + MC method relies on a periodic calculation performed at a lower level of theory to model the crystalline environment. The GIPAW result is then corrected using a single molecule calculation performed at a higher level of theory. The success of the GIPAW + MC method in predicting a range of NMR parameters is a result of the highly local character of the tensors underlying the NMR observable. However, in applications involving strong intermolecular interactions we find that expanding the region treated at the higher level of theory more accurately captures local many-body contributions to the N15 NMR chemical shielding (CS) tensor. We propose alternative corrections to GIPAW which capture interactions between adjacent molecules at a higher level of theory using either fragment or cluster-based calculations. Benchmark calculations performed on N15 and C13 data sets show that these advanced GIPAW-corrected calculations improve the accuracy of chemical shielding tensor predictions relative to existing methods. Specifically, cluster-based N15 corrections show a 24% and 17% reduction in RMS error relative to GIPAW and GIPAW + MC calculations, respectively. Comparing the benchmark data sets using multiple computational models demonstrates that N15 CS tensor calculations are significantly more sensitive to intermolecular interactions relative to C13. However, fragment and cluster-based corrections that include direct hydrogen bond partners are sufficient for optimizing the accuracy of GIPAW-corrected methods. Finally, GIPAW-corrected methods are applied to the particularly challenging NMR spectral assignment of guanosine dihydrate which contains two guanosine molecules in the asymmetric unit.

First Direct Insight into the Local Environment and Dynamics of Water Molecules in the Whewellite Mineral Phase: Mechanochemical Isotopic Enrichment and High-Resolution 17O and 2H NMR Analyses

Calcium oxalate minerals of the general formula CaC₂O₄ ^. xH₂O are widely present in nature and usually associated with pathological calcifications, constituting up to 70-80% of the mineral component of renal calculi. The monohydrate phase (CaC₂O₄ ^.H₂O, COM) is the most stable form, accounting for the majority of the hydrated calcium oxalates found. These mineral phases have been studied extensively via X-ray diffraction and IR spectroscopy and, to a lesser extent, using ¹H, ¹³C, and ⁴³Ca solid-state NMR spectroscopy. However, several aspects of their structure and reactivity are still unclear, such as the evolution from low- to high-temperature COM structures (LT-COM and HT-COM, respectively) and the involvement of water molecules in this phase transition. Here, we report for the first time a ¹⁷O and ²H solid-state NMR investigation of the local structure and dynamics of water in the COM phase. A new procedure for the selective ¹⁷O- and ²H-isotopic enrichment of water molecules within the COM mineral is presented using mechanochemistry, which employs only microliter quantities of enriched water and leads to exchange yields up to ∼30%. ¹⁷O NMR allows both crystallographically inequivalent water molecules in the LT-COM structure to be resolved, while ²H NMR studies provide unambiguous evidence that these water molecules are undergoing different types of motions at high temperatures without exchanging with one another. Dynamics appear to be essential for water molecules in these structures, which have not been accounted for in previous structural studies on the HT-COM structure due to lack of available tools, highlighting the importance of such NMR investigations for refining the overall knowledge on biologically relevant minerals like calcium oxalates.

Solid-state 17O NMR study of α-d-glucose: exploring new frontiers in isotopic labeling, sensitivity enhancement, and NMR crystallography

We report the first “total synthesis” of 17O-labeled d-glucose and its solid-state 17O NMR characterization with unprecedented sensitivity and resolution.

Racing toward Fast and Effective 17O Isotopic Labeling and Nuclear Magnetic Resonance Spectroscopy of N-Formyl-MLF-OH and Associated Building Blocks

Solid-state ¹H, ¹³C, and ¹⁵N nuclear magnetic resonance (NMR) spectroscopy has been an essential analytical method in studying complex molecules and biomolecules for decades. While oxygen-17 (¹⁷O) NMR is an ideal and robust candidate to study hydrogen bonding within secondary and tertiary protein structures for example, it continues to elude many. We discuss an improved multiple-turnover labeling procedure to develop a fast and cost-effective method to ¹⁷O label fluoroenylmethyloxycarbonyl (Fmoc)-protected amino acid building blocks. This approach allows for inexpensive ($0.25 USD/mg) insertion of ¹⁷O labels, an important barrier to overcome for future biomolecular studies. The ¹⁷O NMR results of these building blocks and a site-specific strategy for labeled N-acetyl-MLF-OH and N-formyl-MLF-OH tripeptides are presented. We showcase growth in NMR development for maximizing sensitivity gains using emerging sensitivity enhancement techniques including population transfer, high-field dynamic nuclear polarization, and cross-polarization magic-angle spinning cryoprobes.

Fast and Accurate Electric Field Gradient Calculations in Molecular Solids With Density Functional Theory

Modern approaches for calculating electric field gradient (EFF) tensors in molecular solids rely upon plane-wave calculations employing periodic boundary conditions (PBC). In practice, models employing PBCs are limited to generalized gradient approximation (GGA) density functionals. Hybrid density functionals applied in the context of gauge-including atomic orbital (GIAO) calculations have been shown to substantially improve the accuracy of predicted NMR parameters. Here we propose an efficient method that effectively combines the benefits of both periodic calculations and single-molecule techniques for predicting electric field gradient tensors in molecular solids. Periodic calculations using plane-wave basis sets were used to model the crystalline environment. We then introduce a molecular correction to the periodic result obtained from a single-molecule calculation performed with a hybrid density functional. Single-molecule calculations performed using hybrid density functionals were found to significantly improve the agreement of predicted 17O quadrupolar coupling constants (C q ) with experiment. We demonstrate a 31% reduction in the RMS error for the predicted 17O C q values relative to standard plane-wave methods using a carefully constructed test set comprised of 22 oxygen-containing molecular crystals. We show comparable improvements in accuracy using five different hybrid density functionals and find predicted C q values to be relatively insensitive to the choice of basis set used in the single molecule calculation. Finally, the utility of high-accuracy 17O C q predictions is demonstrated by examining the disordered 4-Nitrobenzaldehyde crystal structure.

Efficient transfer of DNP-enhanced 1 H magnetization to half-integer quadrupolar nuclei in solids at moderate spinning rate

We show herein how the proton magnetization enhanced by dynamic nuclear polarization (DNP) can be efficiently transferred at moderate magic-angle spinning (MAS) frequencies to half-integer quadrupolar nuclei, S ≥ 3/2, using the Dipolar-mediated Refocused Insensitive Nuclei Enhanced by Polarization Transfer (D-RINEPT) technique, in which a symmetry-based SR 4 1 2 recoupling scheme built from adiabatic inversion ¹ H pulses reintroduces the ¹ H-S dipolar couplings, while suppressing the ¹ H-¹ H ones. The use of adiabatic pulses also improves the robustness to offsets and radiofrequency (rf)-field inhomogeneity. Furthermore, the efficiency of the polarization transfer is further improved by using ¹ H composite pulses and continuous-wave irradiations between the recoupling blocks, as well as by manipulating the S satellite transitions during the first recoupling block. Furthermore, in the case of large ¹ H-S dipolar couplings, the D-RINEPT variant with two pulses on the quadrupolar channel results in an improved transfer efficiency. We compare here the performances of this new adiabatic scheme with those of its parent version with single π pulses, as well as with those of PRESTO and CPMAS transfers. This comparison is performed using simulations as well as DNP-enhanced ²⁷ Al, ⁹⁵ Mo, and ¹⁷ O NMR experiments on isotopically unmodified γ-alumina, hydrated titania-supported MoO₃ , Mg(OH)₂ , and l-histidine·HCl·H₂ O. The introduced RINEPT method outperforms the existing methods, both in terms of efficiency and robustness to rf-field inhomogeneity and offset.

Expanding the NMR toolkit for biological solids: oxygen-17 enriched Fmoc-amino acids

We report the solid-state ¹⁷O NMR parameters for five previously uncharacterized N-α-fluoren-9-yl-methoxycarbonyl-O-t-butyl (Fmoc) protected amino acids.

Higher Magnetic Fields, Finer MOF Structural Information: 17O Solid-State NMR at 35.2 T

The spectroscopic study of oxygen, a vital element in materials, physical, and life sciences, is of tremendous fundamental and practical importance. ¹⁷O solid-state NMR (SSNMR) spectroscopy has evolved into an ideal site-specific characterization tool, furnishing valuable information on the local geometric and bonding environments about chemically distinct and, in some favorable cases, crystallographically inequivalent oxygen sites. However, ¹⁷O is a challenging nucleus to study via SSNMR, as it suffers from low sensitivity and resolution, owing to the quadrupolar interaction and low ¹⁷O natural abundance. Herein, we report a significant advance in ¹⁷O SSNMR spectroscopy. ¹⁷O isotopic enrichment and the use of an ultrahigh 35.2 T magnetic field have unlocked the identification of many inequivalent carboxylate oxygen sites in the as-made and activated phases of the metal-organic framework (MOF) α-Mg₃(HCOO)₆. The subtle ¹⁷O spectral differences between the as-made and activated phases yield detailed information about host-guest interactions, including insight into nonconventional O···H-C hydrogen bonding. Such weak interactions often play key roles in the applications of MOFs, such as gas adsorption and biomedicine, and are usually difficult to study via other characterization routes. The power of performing ¹⁷O SSNMR experiments at an ultrahigh magnetic field of 35.2 T for MOF characterization is further demonstrated by examining activation of the MIL-53(Al) MOF. The sensitivity and resolution enhanced at 35.2 T allows partially and fully activated MIL-53(Al) to be unambiguously distinguished and also permits several oxygen environments in the partially activated phase to be tentatively identified. This demonstration of the very high resolution of ¹⁷O SSNMR recorded at the highest magnetic field accessible to chemists to date illustrates how a broad variety of scientists can now study oxygen-containing materials and obtain previously inaccessible fine structural information.

A combined solid-state 17O NMR, crystallographic, and computational study of oxiranes

We report the synthesis and solid-state 17O NMR characterization of three 17O-labeled oxiranes: (2S*,3S*)-2,3-bis(4-nitrophenyl)-[17O]oxirane, (2S*,3R*)-2,3-bis(4-nitrophenyl)-[17O]oxirane, and 2,2,3-triphenyl-[17O]oxirane. In addition, we have determined the crystal structure of (2S*,3R*)-2,3-bis(4-nitrophenyl)oxirane by X-ray crystallography. When the experimentally determined 17O NMR tensors for oxiranes (where the C–O–C bond angle is about 60°) are compared with those for dimethyl ether (where the C–O–C bond angle is 113°) and other R–O–R′ functional groups, we found that the highly constrained geometry of oxiranes results in distinct tensor orientations in the molecular frame of reference. The experimental results are complemented by quantum chemical computations. This study represents the first time that 17O chemical shift and quadrupole coupling tensors are simultaneously determined for oxirane compounds.

17O NMR studies of organic and biological molecules in aqueous solution and in the solid state

This review describes the latest developments in the field of ¹⁷O NMR spectroscopy of organic and biological molecules both in aqueous solution and in the solid state. In the first part of the review, a general theoretical description of the nuclear quadrupole relaxation process in isotropic liquids is presented at a mathematical level suitable for non-specialists. In addition to the first-order quadrupole interaction, the theory also includes additional relaxation mechanisms such as the second-order quadrupole interaction and its cross correlation with shielding anisotropy. This complete theoretical treatment allows one to assess the transverse relaxation rate (thus the line width) of NMR signals from half-integer quadrupolar nuclei in solution over the entire range of motion. On the basis of this theoretical framework, we discuss general features of quadrupole-central-transition (QCT) NMR, which is a particularly powerful method of studying biomolecules in the slow motion regime. Then we review recent advances in ¹⁷O QCT NMR studies of biological macromolecules in aqueous solution. The second part of the review is concerned with solid-state ¹⁷O NMR studies of organic and biological molecules. As a sequel to the previous review on the same subject [G. Wu, Prog. Nucl. Magn. Reson. Spectrosc. 52 (2008) 118-169], the current review provides a complete coverage of the literature published since 2008 in this area.

High-Resolution 17O NMR Spectroscopy of Structural Water

The importance of studying site-specific interactions of structurally similar water molecules in complex systems is well known. We demonstrate the ability to resolve four distinct bound water environments within the crystal structure of lanthanum magnesium nitrate hydrate via 17O solid state nuclear magnetic resonance (NMR) spectroscopy. Using high-resolution multidimensional experiments at high magnetic fields (18.8-35.2 T), each individual water environment was resolved. The quadrupole coupling constants and asymmetry parameters of the 17O of each water were determined to be between 6.6 and 7.1 MHz, 0.83 and 0.90, respectively. The resolution of the four unique, yet similar, structural waters within a hydrated crystal via 17O NMR spectroscopy demonstrates the ability to decipher the unique electronic environment of structural water within a single hydrated crystal structure.

Advances in instrumentation and methodology for solid-state NMR of biological assemblies

Many advances in instrumentation and methodology have furthered the use of solid-state NMR as a technique for determining the structures and studying the dynamics of molecules involved in complex biological assemblies. Solid-state NMR does not require large crystals, has no inherent size limit, and with appropriate isotopic labeling schemes, supports solving one component of a complex assembly at a time. It is complementary to cryo-EM, in that it provides local, atomic-level detail that can be modeled into larger-scale structures. This review focuses on the development of high-field MAS instrumentation and methodology; including probe design, benchmarking strategies, labeling schemes, and experiments that enable the use of quadrupolar nuclei in biomolecular NMR. Current challenges facing solid-state NMR of biological assemblies and new directions in this dynamic research area are also discussed.

Probing Axial Water Bound to Copper in Tutton Salt Using Single Crystal 17O-ESEEM Spectroscopy

Electron spin-echo envelope modulation (ESEEM) signals attributed to axial water bound to Cu²⁺ have been detected and analyzed in Cu(II)-doped ¹⁷O-water-enriched potassium zinc sulfate hexahydrate (Tutton salt) crystals. The magnetic field orientation dependences of low frequency modulations were measured to fit hyperfine and quadrupole coupling tensors of a ¹⁷O ( I = ⁵/₂) nucleus. The hyperfine tensor ( A _xx, A _yy, A _zz: 0.13, 0.23, -3.81 MHz) exhibits almost axial symmetry with the largest value directed normal to the metal equatorial plane in the host structure. Comparisons with quantum chemical calculations position this nucleus about 2.3 Å from the copper. The isotropic coupling (-1.15 MHz) is small and reflects the weak axial water interaction with a d_x2-y2 unshared orbital of copper. The ¹⁷O-water quadrupole interaction parameters ( e² qQ/ h = 6.4 MHz and η = 0.93) are close to the average of those found in a variety of solid hydrates. In addition, the coupling tensor directions correlate very closely with the O8 water geometry, with the maximum quadrupole direction 3° from the water plane normal, and its minimum coupling about 2° from the H-H direction. In almost all previous magnetic resonance ¹⁷O-water studies, the quadrupole tensor orientation was based on theoretical considerations. This work represents one of the few experimental confirmations of its principal axis frame. When Cu²⁺ dopes into the Tutton salt, a Jahn-Teller distortion interchanges the relative long and intermediate metal O7 and O8 bond lengths of the zinc host. Therefore, only those unit cells containing the impurity conform to the pure copper Tutton structure. This study provides further support for this model. Moreover, coupling interactions from distant H₂¹⁷O such as in the present case have important implications in studies of copper enzymes and proteins where substrates have been proposed to displace weakly bound water in the active site.

Direct detection and characterization of bioinorganic peroxo moieties in a vanadium complex by 17O solid-state NMR and density functional theory

Electronic and structural properties of short-lived metal-peroxido complexes, which are key intermediates in many enzymatic reactions, are not fully understood. While detected in various enzymes, their catalytic properties remain elusive because of their transient nature, making them difficult to study spectroscopically. We integrated 17O solid-state NMR and density functional theory (DFT) to directly detect and characterize the peroxido ligand in a bioinorganic V(V) complex mimicking intermediates non-heme vanadium haloperoxidases. 17O chemical shift and quadrupolar tensors, measured by solid-state NMR spectroscopy, probe the electronic structure of the peroxido ligand and its interaction with the metal. DFT analysis reveals the unusually large chemical shift anisotropy arising from the metal orbitals contributing towards the magnetic shielding of the ligand. The results illustrate the power of an integrated approach for studies of oxygen centers in enzyme reaction intermediates.

Zwitterionization of glycine in water environment: Stabilization mechanism and NMR spectral signatures

At physiological conditions, myriads of biomolecules (e.g., amino acids, peptides, and proteins) exist predominantly in the zwitterionic structural form and their biological functions will result in these conditions. However these geometrical structures are inaccessible energetically in the gas phase, and at this point, stabilization of amino-acids in physiological conditions is still under debate. In this paper, the electronic properties of a glycine molecule in the liquid environment were studied by performing a relaxation of the glycine geometry in liquid water using the free energy gradient method combined with a sequential quantum mechanics/molecular mechanics approach. A series of Monte Carlo Metropolis simulations of the glycine molecule embedded in liquid water, followed by only a quantum mechanical calculation in each of them were carried out. Both the local and global liquid environments were emphasized to obtain nuclear magnetic resonance (NMR) parameters for the glycine molecule in liquid water. The results of the equilibrium structure in solution and the systematic study of the hydrogen bonds were used to discard the direct proton transfer from the carboxyl group to the ammonium group of the glycine molecule in water solution. The calculations of the Density Functional Theory (DFT) were performed to study the polarization of the solvent in the parameters of nuclear magnetic resonance of the glycine molecule in liquid water. DFT calculations predicted isotropic chemical changes on the H, C, N, and O atoms of glycine in liquid water solution which agree with the available experimental data.

17O MAS NMR Correlation Spectroscopy at High Magnetic Fields

The structure of two protected amino acids, FMOC-l-leucine and FMOC-l-valine, and a dipeptide, N-acetyl-l-valyl-l-leucine (N-Ac-VL), were studied via one- and two-dimensional solid-state nuclear magnetic resonance (NMR) spectroscopy. Utilizing ¹⁷O magic-angle spinning (MAS) NMR at multiple magnetic fields (17.6-35.2 T/750-1500 MHz for ¹H) the ¹⁷O quadrupolar and chemical shift parameters were determined for the two oxygen sites of each FMOC-protected amino acids and the three distinct oxygen environments of the dipeptide. The one- and two-dimensional, ¹⁷O, ¹⁵N-¹⁷O, ¹³C-¹⁷O, and ¹H-¹⁷O double-resonance correlation experiments performed on the uniformly ¹³C,¹⁵N and 70% ¹⁷O-labeled dipeptide prove the attainability of ¹⁷O as a probe for structure studies of biological systems. ¹⁵N-¹⁷O and ¹³C-¹⁷O distances were measured via one-dimensional REAPDOR and ZF-TEDOR experimental buildup curves and determined to be within 15% of previously reported distances, thus demonstrating the use of ¹⁷O NMR to quantitate interatomic distances in a fully labeled dipeptide. Through-space hydrogen bonding of N-Ac-VL was investigated by a two-dimensional ¹H-detected ¹⁷O R³-R-INEPT experiment, furthering the importance of ¹⁷O for studies of structure in biomolecular solids.

Solid-state 17O NMR study of 2-acylbenzoic acids and warfarin

We report synthesis and solid-state ¹⁷O NMR characterization of four site-specifically ¹⁷O-labeled 2-acylbenzoic acids (2-RC(O)C₆H₄COOH) where R=H and CH₃): 2-[3-¹⁷O]formylbenzoic acid, 2-[1,2-¹⁷O₂]formylbenzoic acid, 2-[3-¹⁷O]acetylbenzoic acid, and 2-[1,2,3-¹⁷O₃]acetylbenzoic acid. In the solid state, both 2-formyl- and 2-acetyl-benzoic acids exist as the cyclic phthalide form each containing a five-membered lactone ring and a cyclic hemiacetal/hemiketal group. Static and magic-angle-spinning ¹⁷O NMR spectra were recorded at 14.1 and 21.1T for these compounds, from which the ¹⁷O chemical shift and nuclear quadrupolar coupling tensors were determined for each oxygen site. These results represent the first time that ¹⁷O NMR tensors are fully characterized for lactone, cyclic hemiacetal, and cyclic hemiketal functional groups. We also report solid-state ¹⁷O NMR data for the cyclic hemiketal group an anticoagulant drug, warfarin. Experimental ¹⁷O NMR tensors in these compounds were compared with computational results obtained with a periodic DFT code BAND.

Structure and Dynamics of Polysaccharides in Plant Cell Walls from Solid-State NMR

Multidimensional high-resolution magic-angle-spinning solid-state NMR (SSNMR) spectroscopy has recently been shown to have the unique capability of revealing the molecular structure and dynamics of insoluble macromolecules in intact plant cell walls. This chapter summarizes the 2D and 3D SSNMR techniques used so far to study cell walls and key findings about cellulose interactions with matrix polysaccharides, cellulose microfibril structure, polysaccharide–protein interactions that are responsible for wall loosening, and polysaccharide–water interactions in the hydrated primary walls. These results provide detailed molecular insights into the structure of near-native plant cell walls, and revise the conventional tethered-network model by suggesting a single-network model for the primary cell wall, which has found increasing support from recent biochemical and biomechanical data.

Unleashing the Potential of 17 O NMR Spectroscopy Using Mechanochemistry

¹⁷ O NMR spectroscopy has been the subject of vivid interest in recent years, because there is increasing evidence that it can provide unique insight into the structure and reactivity of many molecules and materials. However, due to the very poor natural abundance of oxygen-17, ¹⁷ O labeling is generally a prerequisite. This is a real obstacle for most research groups, because of the high costs and/or strong experimental constraints of the most frequently used ¹⁷ O-labeling schemes. Here, we show for the first time that mechanosynthesis offers unique opportunities for enriching in ¹⁷ O a variety of organic and inorganic precursors of synthetic interest. The protocols are fast, user-friendly, and low-cost, which makes them highly attractive for a broad research community, and their suitability for ¹⁷ O solid-state NMR applications is demonstrated.

Solid-State 17 O NMR Reveals Hydrogen-Bonding Energetics: Not All Low-Barrier Hydrogen Bonds Are Strong

While NMR and IR spectroscopic signatures and structural characteristics of low-barrier hydrogen bond (LBHB) formation are well documented in the literature, direct measurement of the LBHB energy is difficult. Here, we show that solid-state ¹⁷ O NMR spectroscopy can provide unique information about the energy required to break a LBHB. Our solid-state ¹⁷ O NMR data show that the HB enthalpy of the O⋅⋅⋅H⋅⋅⋅N LBHB formed in crystalline nicotinic acid is only 7.7±0.5 kcal mol^-1 , suggesting that not all LBHBs are particularly strong.

(17)O NMR Investigation of Water Structure and Dynamics

The structure and dynamics of the bound water in barium chlorate monohydrate were studied with (17)O nuclear magnetic resonance (NMR) spectroscopy in samples that are stationary and spinning at the magic-angle in magnetic fields ranging from 14.1 to 21.1 T. (17)O NMR parameters of the water were determined, and the effects of torsional oscillations of the water molecule on the (17)O quadrupolar coupling constant (CQ) were delineated with variable temperature MAS NMR. With decreasing temperature and reduction of the librational motion, we observe an increase in the experimentally measured CQ explaining the discrepancy between experiments and predictions from density functional theory. In addition, at low temperatures and in the absence of (1)H decoupling, we observe a well-resolved (1)H-(17)O dipole splitting in the spectra, which provides information on the structure of the H2O molecule. The splitting arises because of the homogeneous nature of the coupling between the two (1)H-(17)O dipoles and the (1)H-(1)H dipole.

²H Solid-State NMR Analysis of the Dynamics and Organization of Water in Hydrated Chitosan

Understanding water–biopolymer interactions, which strongly affect the function and properties of biopolymer-based tissue engineering and drug delivery materials, remains a challenge. Chitosan, which is an important biopolymer for the construction of artificial tissue grafts and for drug delivery, has attracted extensive attention in recent decades, where neutralization with an alkali solution can substantially enhance the final properties of chitosan films cast from an acidic solution. In this work, to elucidate the effect of water on the properties of chitosan films, we investigated the dynamics and different states of water in non-neutralized (CTS-A) and neutralized (CTS-N) hydrated chitosan by mobility selective variable-temperature (VT) ²H solid-state NMR spectroscopy. Four distinct types of water exist in all of the samples with regards to dynamic behavior. First, non-freezable, rigid and strongly bound water was found in the crystalline domain at low temperatures. The second component consists of weakly bound water, which is highly mobile and exhibits isotropic motion, even below 260 K. Another type of water undergoes well-defined 180° flips around their bisector axis. Moreover, free water is also present in the films. For the CTS-A sample in particular, another special water species were bounded to acetic acid molecules via strong hydrogen bonding. In the case of CTS-N, the onset of motions of the weakly bound water molecules at 260 K was revealed by ²H-NMR spectroscopy. This water is not crystalline, even below 260 K, which is also the major contribution to the flexibility of chitosan chains and thus toughness of materials. By contrast, such motion was not observed in CTS-A. On the basis of the ²H solid-state NMR results, it is concluded that the unique toughness of CTS-N mainly originates from the weakly bound water as well as the interactions between water and the chitosan chains.

Oxygen-17 NMR spectroscopy of water molecules in solid hydrates

17O solid-state NMR studies of waters of hydration in crystalline solids are presented. The 17O quadrupolar coupling and chemical shift (CS) tensors, and their relative orientations, are measured experimentally at room temperature for α-oxalic acid dihydrate, barium chlorate monohydrate, lithium sulfate monohydrate, potassium oxalate monohydrate, and sodium perchlorate monohydrate. The 17O quadrupolar coupling constants (CQ) range from 6.6 to 7.35 MHz and the isotropic chemical shifts range from –17 to 19.7 ppm. The oxygen CS tensor spans vary from 25 to 78 ppm. These represent the first complete CS and electric field gradient tensor measurements for water coordinated to metals in the solid state. Gauge-including projector-augmented wave density functional theory calculations overestimate the values of CQ, likely due to librational dynamics of the water molecules. Computed CS tensors only qualitatively match the experimental data. The lack of strong correlations between the experimental and computed data, and between these data and any single structural feature, is attributed to motion of the water molecules and to the relatively small overall range in the NMR parameters relative to their measurement precision. Nevertheless, the isotropic chemical shift, quadrupolar coupling constant, and CS tensor span clearly differentiate between the samples studied and establish a ‘fingerprint’ 17O spectral region for water coordinated to metals in solids.

Solid-State ¹⁷O NMR studies of organic and biological molecules: Recent advances and future directions

This Trends article highlights the recent advances published between 2012 and 2015 in solid-state (17)O NMR for organic and biological molecules. New developments in the following areas are described: (1) new oxygen-containing functional groups, (2) metal organic frameworks, (3) pharmaceuticals, (4) probing molecular motion in organic solids, (5) dynamic nuclear polarization, and (6) paramagnetic coordination compounds. For each of these areas, the author offers his personal views on important problems to be solved and possible future directions.

Structural Insights into Bound Water in Crystalline Amino Acids: Experimental and Theoretical (17)O NMR

We demonstrate here that the (17)O NMR properties of bound water in a series of amino acids and dipeptides can be determined with a combination of nonspinning and magic-angle spinning experiments using a range of magnetic field strengths from 9.4 to 21.1 T. Furthermore, we propose a (17)O chemical shift fingerprint region for bound water molecules in biological solids that is well outside the previously determined ranges for carbonyl, carboxylic, and hydroxyl oxygens, thereby offering the ability to resolve multiple (17)O environments using rapid one-dimensional NMR techniques. Finally, we compare our experimental data against quantum chemical calculations using GIPAW and hybrid-DFT, finding intriguing discrepancies between the electric field gradients calculated from structures determined by X-ray and neutron diffraction.